Method of forming diffusion coatings

ABSTRACT

A method of diffusing elements into a ferrous substrate using molten lead as the transport medium for the elements being diffused. Such diffusion coatings containing chromium, cobalt, manganese, and other elements that improve corrosion resistance of ferrous materials have considerable utility.

ausch et a1.

[451 Jan. 22, mm

METHOD OF FORMING DIFFUSION COATINGS Inventors: John J. Rausch, Antioch; Ray J.

Van Thyne, Oak Lawn, both of I11.

Assignee: Surfalloy Corporation, Chicago, 111.

Filed: Nov. 12, 1971 Appl. No.: 198,403

Related US. Application Data Continuation-impart of Ser. No. 768,187, Oct. 16, 1968, Pat. No. 3,620,816.

US. Cl...... 117/114 R, 117/114 A, 117/114 B, 117/114 C, 117/131- Int. Cl. C23c 9/00 Field ofSearch ..1l7/l14 R, 114 A,114B, 117/114 C,l18,131.135.1;148/15.5,15, 31.5, 6.11

Primary ExaminerRalph S. Kendall Attorney, Agent, or Firm-A1bert Siege] [57] ABSTRACT A method of diffusing elements into a ferrous substrate using molten lead as the transport medium for the elements being diffused. Such diffusion coatings containing chromium, cobalt, manganese, and other elements that improve corrosion resistance of ferrous materials have considerable utility.

13 Claims, No Drawings METHOD or it: i ll nrrrusron commas CROSS REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION In our copending U.S. Pat. application (Ser. No. 768,187) now U.S. Pat. No. 3,620,816 there is described a diffusion coating process in which a metal article is surface alloyed by contacting the article with molten lead containing one or more diffusing elements. This process particularly provides chromium impregnated (chromized), aluminized or titanized ferrous articles which have good resistance to corrosion or oxidation. Furthermore the teachings of the patent show the desirability of incorporating certain elements codiffused with chromium. Such elements include aluminum, arsenic, beryllium, columbium, germanium, molybdenum, phosphorous, silicon, tin, titanium, vanadium, tantalum, tungsten, and nickel. These elements aid in avoiding undesireable interactions between chromium and carbon and result in more rapid surface alloying or improved corrosion or oxidation resistance.

We have found furthermore that the process can be used to diffuse elements other than chromium, aluminium, and titanium. The elements that were cited in our copending application as being co-diffuseable with chromium can be diffused alone or in combination in the absence of chromium. Furthermore, elements such as cobalt, manganese, copper, zirconium, hafnium, rhenium, magnesium, antimony, selenium, tellurium, yttrium, the rare earths, and platinum group metals can be transferred to a ferrous substrate in a lead bath. In fact, any element soluble in lead that improves the properties of ferrous materials can be transferred by this process. We have also found that certain of these elements when codiffused with chromium, produce desireable effects and result in corrosion resistant surface layers. This is notably true of the elements cobalt and manganese.

' Accordingly, the principal objective of our invention is to provide a novel method for diffusion coating ferrous articles. This includes the process of cobaltizing, siliconizing, and other surface alloying, singly and in combination, of the elements described above by transfer of said elements through a molten lead bath. A further objective is to provide a novel process for chromizing in molten lead wherein other elements such as cobalt, manganese, and the abovementioned are included in the lead.

DESCRIPTION OF THE INVENTION To carry out this invention we prefer to use a closed system in which all of the ingredients necessary to carry out the reaction, along with the ferrous parts to be diffusion coated are sealed under vacuum or with a small amount of residual inert gas. There are of course other methods for conducting this process and in fact the reactions can be carried out in the presence of small amounts of air or oxygen.

The molten lead bath should preferably contain adequate amounts of the transfer elements to saturate the bath therewith and to provide a source of these elements for continuous resaturation of that which is consumed during the surface alloying process. The elements are added to the lead bath in any convenient soluble form such as elemental powder or granules or as an alloy. The process can be carried out over a wide range of temperatures, however, we prefer to operate in the range from the melting point of lead to 2,200F.

Our invention may be further understood by reference to the following examples thereof:

EXAMPLE 1 We prepared a materials charge consisting of 800 grams of lead, 12 grams of chromium, and 3 grams of cobalt. Such charge and test pieces consisting of Armco iron and decarburized C1108 steel were sealed in an evacuated steel tube and then uniformly heated at 1,950F for 7 hours. The tube was shaken vigorously every 15 minutes to facilitate homogenization of the bath. After the diffusion coating reaction the specimens were cleaned to remove residual lead from the surface. Metallographic examination showed that a 3.0 mil diffusion coating, that was resistant to attack by HNO was formed. The micro-hardness of this zone, measured on a cross-section with a diamond pyramid indentor at a 50 gram load, varied from 186 DPN at the surface to 174 DPN at a depth of 2.5 mils. The hardness of the iron substrate was 97 DPN.

A specimen of the surface alloyed zone was obtained by sectioning treated samples and removing the iron substrate by immersion in hot nitric acid. The residual sample was dissolved for analysis thereby representing an average concentration of the graded alloyed zone. Chemical analysis, by atomic absorption, showed the following concentrations in the diffusion coating:

chromium 24.5 w/o cobalt 5.7 w/o The ratio of chromium to cobalt in the diffusion 'zone is about the same as that in the original materials charge.

EXAMPLE 2 A materials charge consisting of 250 grams of lead, 3 grams of chromium, and 3 grams of cobalt was prepared. Armco iron specimens were run with this charge under conditions similar to those in the previous example, except that the processing time was 4 hours rather than 7. Metallographic examination showed that a 1.6 mil diffusion coating, that was resistant to attack by HNO was formed. The coating was composed of two layers. The outermost, having a thickness of 1.0 mils had an average microhardness of 1,025 DPN; the inner layer was softer, having an average hardness of DPN.

EXAMPLE 3 The experiment described above was repeated except that the reaction was carried out at 2,05 0F rather than 1,950F. Metallographic examination showed that a 2 layer coating was again formed. The outer layer, 1.7 mils thick had an average microhardness of 880 DPN; the inner layer 2.0 mils thick, had a microhardness of 260 DPN.

EXAMPLE 4 A number of runs were made using a charge of 200 grams lead, 3 grams cobalt granules (mesh size +l/ 16-1/8 in.) and various ferrous materials having dimensions approximately A in. dia. XVz in. long. The following data were obtained:

previously decarburized at I500F-24 hr. in an atmosphere of N,-IOH, saturated with water at +75F.

When materials such as the carbon containing steels listed above are chromized a hard carbide layer is formed at the surface. It will be observed, that unlike chromizing, there is no hard carbide layer formed at the surface when such steels are cobaltized. The presence of cobalt in these surface layers should increase the elevated temperature strength and oxidation resistance of such materials.

A specimen of Cl018 steel cobaltized at 1,950F was oxidized in air at 1,200F for 4 hrs. A tight, adherent oxide layer was formed which did not spall. The specimen showed a weight gain of 1.2 mg/cm as a result of this exposure.

We have found the corrosion resistance of the cobaltized Armco iron to be excellent in concentrated hydrochloric acid at room temperature. A specimen of this type, having a surface area of 1.2 cm showed no detectable weight loss after 30 minutes when weighed to an accuracy of :01 mg.

EXAMPLE A materials charge consisting of 200 grams of lead, 3 grams of nickel sheet and four decarburized 412O A inch thread diameter and threads per inch) mild steel nuts was prepared. This was run under conditions similar to those described above at 1,500F for 8 hours. Metallographic examination of the specimens showed an outer austenitic layer, 0.7 mil thick, which was not attacked by Nital etchant (5% HNO;, in ethanol). Beneath the outer zone there was a region 1.2 mils thick consisting of a mixture of austenite and nickel-enriched ferrite grains.

EXAMPLE 6 A materials charge consisting of 200 grams of lead,

3 grams of manganese granules (+l/l6-1/8 in. mesh) and 2 specimens ofCl0l8 steel, )4; in. dia. X in. long,

was prepared and run at l,950F for 4 hours. A two layered diffusion coating was formed, the outer layer having a thickness of 0.2 mil and the inner 0.3 mil.

EXAMPLE 8 An experiment similar to that in example 7 was run in which 3 grams of chromium granules was included along with 3 grams of manganese granules of similar mesh size. A two layered diffusion coating was formed, each layer having a thickness of 0.6 mil. The outer layer was not attacked by Nita].

Many other examples of diffusion formed coatings such as siliconizing and others could be given, however it is obvious that essentially any metal soluble in lead can be transferred to form a diffusion coating by our process.

In addition to the elements claimed in our pending application, we have found that others can be codiffused with chromium and aluminium. These include the group cobalt, manganese, copper, zirconium, hafnium, rhenium, zinc, magnesium, antimony, selenium, tellurium, yttrium, the rare earth metals, and the metals of the platinum group. Thus, this group may be codiffused with the previously claimed group which includes titanium, arsenic, beryllium, columbium, germanium, molybdenum, phosphorous, silicon, tin, vanadium, tantalum, tungsten, and nickel; or all three groups may be codiffused. It is to be understood that any of the elements can be diffused as a single specie, or with other elements within its group.

We have found that our process results in very uniform coatings that can be controlled and repreoduced over precise limits. The throwing power of the process is excellent as evidenced by our ability to uniformly coat complex geometries, recesses and blind holes. We have successfully diffusion coated ferrous parts having a blind hole 0.030 in. dia. X 0.30 in. deep. Even more extreme geometries should offer no diff culty in this respect.

While the foregoing description of the various embodiments of our invention is directed principally to. the use of a molten lead bath as the alloying material transfer medium, it certainly will be understood by those skilled in this art that the molten lead need not be in the form of a molten pool per se but it is also possible to carry out our process, for example, by painting the substrate with a lead-alloying addition compelx and then heating the thus coated material to surface alloy. Furthermore, our process certainly has applicability in the alloying of non-ferrous metals and alloys. It should also be noted that the bath need not be completely lead--it may contain small amounts of other inert diluents in addition to the material acting as the alloying agent.

It will be understood that various modifications and variations may be affected without departing from the spirit or scope of the novel concepts of our invention.

We claim as our invention:

1. The process of diffusion coating a ferrous base substrate which includes the steps of contacting said substrate with a molten alloy bath consisting essentially of lead and at least one diffusing element from each of three groups of elements wherein:

Group 1 is chromium, aluminum and titanium;

Group 2 is beryllium, columbium, germanium, molybdenum, phosphorus, silicon, tin, vanadium, tantalum, tungsten and nickel;

Group 3 is cobalt, manganese, copper, zirconium,

hafnium, rhenium, zinc, magnesium, selenium, tellurium, yttrium, the rare earth metals, and the metals of the platinum group.

and diffusing said elements into said substrate.

2. The process as defined in claim 1 in which only chromium of group 1 is present and one or more elements from each of group 2 and group 3 are present.

3. The process as defined in claim I in which only aluminum of group 1 is present and one or more elements from each of group 2 and group 3 are present.

4. The process of diffusion coating a ferrous base substrate which includes the steps of contacting said substrate with a molten alloy bath consisting essentially of lead and at least one diffusing element from each of two groups of elements wherein: v

Group 1 is chromium, aluminum and titanium;

Group 2 is cobalt, manganese, copper, zirconium,

hafnium, rhenium, zinc, magnesium, selenium, tellurium, yttrium, the rare earth metals, and the meatls of the platinum group,

and diffusing said elements into said substrate.

5. The process as defined in claim 4 in which only chromium of Group 1 is present and one or more elements of Group 2 are present.

6. The process as defined in claim 4 in which only chromium of Group 1 is present and only cobalt of Group 2 is present.

7. The process as defined in claim 4 in which only chromium of Group 1 is present and only manganese of Group 2 is present,

8. The process of diffusion coating a ferrous base substrate which includes the steps of contacting said substrate with a molten alloy bath consisting essentially of lead and one or more elements from the group beryllium, columbium, germanium, molybdenum, phosphorus, silicon, tin, vanadium, tantalum, tungsten, and nickel, and diffusing said elements into said substrate.

9. The process as defined in claim 8 in which only silicon is present.

10. The process-of diffusion coating a ferrous base substrate which includes the steps of contacting said substrate with a molten alloy bath consisting essentially of lead and one or more elements from the group cobalt, manganese, copper, zirconium, hafnium, rhenium, zinc, magnesium, selenium, tellurium, yttrium, the rare earth metals, and the metals of the platinum group and diffusing said elements into said substrate.

111. The process as defined in claim 10 in which only cobalt is present.

12. The process of diffusion coating a ferrous base substrate which includes the steps of contacting said substrate with a molten alloy bath consisting essentially of lead and at least one diffusing element from each of two groups of elements wherein:

Group 1 is beryllium, columbium, germanium, molybdenum, phosphorus, silicon, tin, vanadium, tantalum, tungsten and nickel;

Group 2 is cobalt, manganese, copper, zirconium,

hafnium, rhenium, zinc, magnesium, selenium, tellurium, yttrium, the rare earth metals, and the metals of the platinum group, and diffusing said elements into said substrate.

13. The process of diffusion coating a ferrous base substrate which includes the steps of contacting said substrate with a molten alloy bath consisting essentially of lead and diffusing elements of titanium and one or more elements from the group beryllium, columbium, germanium, molybdenum, phosphorus, silicon, tin, vanadium, tantalum, tungsten, and nickel and diffusing said elements into said substrate. 

2. The process as defined in claim 1 in which only chromium of group 1 is present and one or more elements from each of group 2 and group 3 are present.
 3. The process as defined in claim 1 in which only aluminum of group 1 is present and one or more elements from each of group 2 and group 3 are present.
 4. The process of diffusion coating a ferrous base substrate which includes the steps of contacting said substrate with a molten alloy bath consisting essentially of lead and at least one diffusing element from each of two groups of elements wherein: Group 1 is chromium, aluminum and titanium; Group 2 is cobalt, manganese, copper, zirconium, hafnium, rhenium, zinc, magnesium, selenium, tellurium, yttrium, the rare earth metals, and the meatls of the platinum group, and diffusing said elements into said substrate.
 5. The process as defined in claim 4 in which only chromium of Group 1 is present and one or more elements of Group 2 are present.
 6. The process as defined in claim 4 in which only chromium of Group 1 is present and only cobalt of Group 2 is present.
 7. The process as defined in claim 4 in which only chromium of Group 1 is present and only manganese of Group 2 is present.
 8. The process of diffusion coating a ferrous base substrate which includes the steps of contacting said substrate with a molten alloy bath consisting essentially of lead and one or more elements from the group beryllium, columbium, germanium, molybdenum, phosphorus, silicon, tin, vanadium, tantalum, tungsten, and nickel, and diffusing said elements into said substrate.
 9. The process as defined in claim 8 in which only silicon is present.
 10. The process of diffusion coating a ferrous base substrate which includes the steps of contacting said substrate with a molten alloy bath consisting essentially of lead and one or more elements from the group cobalt, manganese, copper, zirconium, hafnium, rhenium, zinc, magnesium, selenium, tellurium, yttrium, the rare earth metals, and the metals of the platinum group and diffusing said elements into said substrate.
 11. The process as defined in claim 10 in which only cobalt is present.
 12. The process of diffusion Coating a ferrous base substrate which includes the steps of contacting said substrate with a molten alloy bath consisting essentially of lead and at least one diffusing element from each of two groups of elements wherein: Group 1 is beryllium, columbium, germanium, molybdenum, phosphorus, silicon, tin, vanadium, tantalum, tungsten and nickel; Group 2 is cobalt, manganese, copper, zirconium, hafnium, rhenium, zinc, magnesium, selenium, tellurium, yttrium, the rare earth metals, and the metals of the platinum group, and diffusing said elements into said substrate.
 13. The process of diffusion coating a ferrous base substrate which includes the steps of contacting said substrate with a molten alloy bath consisting essentially of lead and diffusing elements of titanium and one or more elements from the group beryllium, columbium, germanium, molybdenum, phosphorus, silicon, tin, vanadium, tantalum, tungsten, and nickel and diffusing said elements into said substrate. 